1. Field
This disclosure relates to super-resolution optical microscopy, especially beneficial for investigation of structures fabricated in semiconductor materials.
2. Related Art
Various annular illumination and imaging are used in the art for scientific investigation. For example, sub-diffraction-limited (i.e. super-resolution) optical microscopy is used in the art of testing and debug of microchips. Such system generally use laser for illumination and high numerical aperture, e.g., solid immersion lens (SIL), to obtain the high resolution.
FIG. 1A illustrates a conventional confocal/laser signal injection microscope. The arrangement consists of a computer 100 and an illumination source 105, such as a laser source. For microchip investigation the laser source 105 may be pulsed or continuous-wave (CW) laser providing beam of wavelength, e.g., 1064 nm or 1340 nm. Confocal optics 110 shapes the beam and directs it onto a scanner, e.g., galvonometer-based scanning mirrors 120 and 125, so as to scan the area of interest through a high numerical aperture objective lens 130 and a solid immersion lens 135. In this example, the back aperture of objective lens 130 is clear (i.e. free of obstructions). This facilitates efficient optical power transmission as well as sufficient resolving capabilities since the high numerical aperture (non-paraxial) components of the incident optical wavefront can contribute towards the interrogation of detailed spatial content from the area of interest.
The lateral spatial resolution performance of these current systems can be enhanced through the use of custom pupil-plane transmission filters (i.e. annular/phase apertures) and/or through vectorial tailoring of the incident electric-field (i.e. through polarization control). An example is illustrated in FIG. 1B. The system of FIG. 1B is similar to that of FIG. 1A, except that a binary amplitude annular aperture 175 is introduced in the beam path. Aperture 175 excludes any low numerical aperture paraxial rays from contributing towards the resulting image, and thus enables a further enhancement of the system's imaging capabilities. As shown in the callout, the annular aperture blocks light rays in the center of the beam, and the amount of blockage can be selected by enlarging or reducing the diameter of the center blocking mask.
Although the above described resolution-enhancements are of significant importance, particularly to the optoelectronic evaluation of nanoscale structures, the use of an annular aperture introduces several system implementation/optimization concerns. For example, the use of an annular aperture leads to a reduced signal being injected/collected due to the blocking aperture. This restriction is of immediate concern since a significant reduction in the transmitted optical power to the device will result in a severely reduced imaging contrast and/or a depleted electrical signal level measured directly from the sample. The power reduction due to the aperture can be overcome by increasing the laser power. However, such a solution leads to other problems, such as heating of the optical elements by the high power laser.
Aperture heating effects may be deleterious to the imaging/probing performance of the system due to thermal expansion effects. Excessive heating in a concentrated area will cause opto-mechanical components in that location to increase in physical size, resulting in a potential disruption to the pre-aligned, and optimized, optical path. It should be noted here that laser-induced heating effects are of particular importance in CW laser imaging and probing investigations using above normal operating powers since the average optical power incident on the component in question will be significantly increased. In order to determine the laser power and/or local temperature required to manifest such negative thermal effects, one must consider the incident optical wavelength used (typically 1064 nm or 1340 nm), the material composition of the opto-mechanical component (for example, protected gold), the corresponding coefficient of thermal expansion of that material (for protected gold this value is 14.2 μm/m/K), the material's thermal conductivity (for protected gold this value is 318 W/m/K) as well as the absorption coefficient of the material (for protected gold this value is 8.69×105/cm at 1064 nm and 8.339×105/cm at 1340 nm). Once these values are understood it is then possible to determine the thermal limitations. For example, one may prefer for the aperture component to be highly absorbing or highly reflecting within their particular configuration. With regard to an absorbing component, it would be beneficial to select a material which has a high absorption coefficient but low thermal expansion/conductivity coefficient in order to efficiently contain the incident optical power within the material without suffering an increase in physical volume or the transfer of excess heat to other mechanical components/mounts. On the other hand, if the aperture was designed to be highly reflective, the material considerations will be tailored to address this issue (e.g. facilitate optimum reflectivity—protected gold is >98% reflective from the visible spectra through to the near IR at normal incidence). There will be limited absorption and hence thermally-induced expansion issues since the majority of the incident optical power will not penetrate the aperture. However, significant back-reflection considerations must then be addressed. These back-reflections could result in a number of detrimental effects; for example, laser source damage and poor imaging performance due to having large background signal incident on the photo detectors.
Another issue that needs to be addressed is multiple back-reflections from the sample, especially if the aperture has a reflective underside. Sample back-reflections may be captured through the transmission region of the aperture and disrupt the imaging performance of the system. Also, in optical probing mode, these back-reflections may be absorbed at different locations across the sample and cause evaluation degradation through photoelectric waveform acquisition, such as in Laser Voltage Probing, or critical timing path analysis, such as in Laser Assisted Device Alteration.
Current super-resolution techniques employing such pupil-function engineered technologies provide no consideration to the negative effects described above. Accordingly, a solution is needed to enable super-resolution microscopy without leading to the problems described above.